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1
Reflectance Spectroscopy of Chromium-bearing Spinel with Application to
2
Recent Orbital Data from the Moon
3
K.B. Williams, C.R.M. Jackson, L.C. Cheek, K.L. Donaldson Hanna, S.W. Parman, C.M.
4
Pieters, M.D. Dyar, T.C. Prissel
5
6
Abstract
7
Visible to near-infrared (V-NIR) remote sensing observations have identified spinel in a
8
variety of locations and lithologies on the Moon. Experimental studies have quantified
9
the FeO content of these spinels (Jackson et al., 2014), however the chromite
10
component is not well constrained. Here we present compositional and spectral
11
analyses of spinel synthesized with varying chromium contents at lunar-like oxygen
12
fugacity (fO2). Reflectance spectra of the chromium-bearing synthetic spinels (Cr# 1-29)
13
have a narrow (~130 nm wide) absorption feature centered at ~550 nm. The 550 nm
14
feature, attributed to octahedral Cr3+, is present over a wide range in iron content (Fe#
15
8-30) and its strength positively correlates with spinel chromium content
16
[ ln(reflectancemin) = - 0.0295 Cr# - 0.3708 ]. Our results provide laboratory
17
characterization for the visible to near-infrared (V-NIR) and mid-infrared (mid-IR)
18
spectral properties of spinel synthesized at lunar-like fO2. The experimentally
19
determined calibration constrains the Cr# of spinels in the lunar Pink Spinel
20
Anorthosites to low values, potentially Cr# < 1. Furthermore, the results suggest the
21
absence of a 550 nm feature in remote spectra of the Dark Mantle Deposits at Sinus
22
Aestuum precludes the presence of a significant chromite component. Combined, the
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23
observation of low chromium spinels across the lunar surface argues for large
24
contributions of anorthositic materials in both plutonic and volcanic rocks on the Moon.
25
Introduction
26
Recent analyses of Chandrayaan-1 Moon Mineralogy Mapper (M3) and SELENE
27
Kaguya Spectral Profiler (SP) orbital data have identified spinel-bearing lithologies
28
across the lunar surface (e.g., Sunshine et al., 2010; Pieters et al., 2011; Dhingra et al.,
29
2011a, 2011b; Yamamoto et al., 2013; Pieters et al., 2014; Sunshine et al., 2014).
30
Detections of these previously unresolved surface components provide new insight into
31
the petrogenesis and evolution of the lunar crust.
32
Most remotely detected spinels are Mg- and Al-rich (“Mg-spinel”) and are thought
33
to be mixed with high proportions of plagioclase based on their prominent 2000 nm
34
absorption feature and relatively high albedos. This new rock type has been termed
35
“pink spinel anorthosite” (PSA, e.g. Taylor and Pieters, 2013; Pieters et al., 2014) and
36
has been found globally (Pieters et al. 2010, 2011, 2013, 2014; Sunshine et al., 2010;
37
Dhingra et al. 2011a, 2011b; Lal et al., 2011, 2012; Dhingra and Pieters, 2011; Kaur et
38
al., 2012, 2013a, 2013b; Bhattacharya et al., 2012, 2013; Donaldson Hanna, 2013;
39
Yamamoto et al., 2013; Sun et al. 2013; Kaur and Chauhan, 2014). Additionally, unique
40
low-albedo regions identified within the Sinus Aestuum pyroclastic deposits are
41
suggested to be rich in Fe- or Cr-rich spinel due to their spectral signature near 1000
42
nm (Sunshine et al., 2010, 2014; Yamamoto et al., 2013). The specific nature of each
43
spinel-bearing lithology remains uncertain and several petrogenetic models have been
44
proposed, including melt-rock reaction and impact melting (Gross and Treiman, 2011;
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45
Yamamoto et al., 2013; Prissel et al., 2014, Gross et al., 2014). Constraining the
46
composition of the remotely sensed lunar spinels is fundamental in identifying plausible
47
formation mechanisms.
48
Previous laboratory investigations have typically focused on characterizing
49
terrestrial spinels (e.g., Cloutis et al., 2004). Spectral comparison with terrestrial spinels
50
suggests PSA lithologies contain Mg-spinel similar to those found in pink spinel
51
troctolites. However, terrestrial spinels form under more oxidizing conditions than lunar
52
spinels, leading to relatively high ferric iron contents. Ferric iron in terrestrial spinels is
53
expressed across the spectral range of M3 and SP, obscuring the connection to lunar
54
spinels (Figure 1). In this context, we conducted the current study to explore the
55
spectral effects of chromium on well-characterized lunar analog spinel.
56
In contrast to the Mg-, Al-rich composition inferred for the remotely sensed
57
spinels, Apollo samples and lunar meteorites mostly contain Cr- and Fe-rich spinels
58
(Haggerty, 1971, 1972, 1973, 1977). A small number of samples (e.g., spinel troctolites,
59
Mg-suite samples) contain spinels similar to those remotely detected. The apparent
60
overrepresentation of Mg-spinels in remote sensing observations may be due to the
61
ease of detecting Mg-spinels relative to Cr-, Fe-rich spinels. Given nearly all lunar spinel
62
samples are chromium bearing (up to 55 wt% Cr2O3), remotely sensed lunar spinels are
63
expected to also contain above trace levels of chromium.
64
Chromium in spinel is spectrally active across the visible to near-infrared
65
wavelengths (V-NIR). In general, spinel spectra are characterized by a pair of strong,
66
broad absorptions near 2000 and 2400 nm attributed to Fe2+ in tetrahedral coordination
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67
(Figure 1; Cloutis et al., 2004; Jackson et al., 2014). Measureable spectral variations,
68
such as the manifestation of subtle absorptions just short of 1000 nm, reflect variations
69
in Fe# (Fe/(Mg+Fe)*100) and Cr# (Cr/(Cr+Al)*100) of spinel, as discussed below.
70
Chromites, the iron-chromium (octahedral Cr3+) endmember of spinel group minerals,
71
are characterized by increasingly strong absorptions into the visible wavelengths and, in
72
many cases, a distinct feature at 1300 nm attributed to tetrahedral Cr2+(Cloutis et al.,
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2004).
74
Laboratory spectroscopic analyses of spinel have also identified a narrow
75
absorption centered at 550 nm (see Figure 1) that has been attributed to octahedral
76
Cr3+ in spinel (Mao and Bell, 1975, Cloutis et al., 2004). This 550 nm absorption is
77
commonly weaker than the chromium absorption at 1300 nm, but it has been previously
78
identified in spectra of lunar samples, such as 70002,7 (Mao and Bell, 1975). The strong
79
preference of Cr3+ for octahedral crystallographic sites in spinel may influence the
80
ordering of other spectrally active cations in the mineral lattice (Navrotsky and Kleppa,
81
1967), such as the ratio of octahedral and tetrahedral Fe2+, and thus, may indirectly
82
affect a range of V-NIR spectral properties of spinel. Though the 550 nm feature
83
appears in laboratory spectra of both terrestrial and lunar spinels, it has yet to be
84
identified in the spectra of remotely sensed spinels.
85
Laboratory studies indicate at FeO contents ≥5 wt%, spectra of synthetic spinels
86
display short wavelength absorption features near 1000 nm related to octahedral Fe2+
87
(Jackson et al., 2014). Low FeO abundances (<5 wt%) are therefore expected for the
88
PSA lithology because remote sensing observations of these regions on the Moon lack
89
observable absorption bands in the 1000 nm spectral range. In contrast, Sinus Aestuum
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spinel reflectance spectra display relatively strong absorptions near 600-700, 900-1000,
91
and 1350 nm, suggesting that the Sinus Aestuum spinels are higher in iron and/or
92
chromium abundance than the PSA spinels (Sunshine et al., 2010, 2014; Yamamoto et
93
al., 2013).
94
The effect of chromium on reflectance properties within the 550-1000 nm spectral
95
range in the absence of abundant Fe3+ has not been experimentally studied at lunar
96
oxygen fugacities. The short wavelengths of terrestrial spinel spectra are dominated by
97
the presence of Fe3+ absorptions (Cloutis et al., 2004), which should be minimal in lunar
98
samples due to the low lunar fO2. In this study, we synthesize chromium- and iron-
99
bearing spinels at lunar-like oxygen fugacity (fO2) in order to determine the influence of
100
chromium content on the spectral characteristics of spinel on the Moon. Identifying the
101
spectral reflectance features of chromium in lunar spinel, as well as constraining the
102
compositions at which the features exist, allows for estimation of the amount of
103
chromium in the spinel-bearing lithologies on the Moon. Our results provide a calibration
104
for interpreting spinel compositions from remote spectral observations and suggest
105
spinels in the PSA lithology have a low Cr#, potentially less than 1.
106
107
Methods
108
Mineral synthesis
109
Aluminate spinels of varying chromium content were synthesized by mixing
110
approximately stoichiometric proportions of reagent-grade oxide powders (MgO, Fe2O3,
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Al2O3, and Cr2O3). Oxide powders were homogenized using a mortar and pestle, then
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112
pressed into 1 cm diameter pellets and placed atop zirconia beads all within a low,
113
rectangular alumina crucible. Samples were then sintered in a horizontal gas-mixing
114
furnace for 72 hours at an oxygen fugacity corresponding to 1 log unit below the iron-
115
wüstite buffer at 1450°C.
116
A series of spinels (CrSp1-8) was synthesized with varying chromium content in
117
order to investigate the effect of chromium on spinel spectra. Eight different starting
118
compositions with Cr# varying from 1 to 29 and a fixed iron content (~5 wt% FeO) were
119
produced to create the CrSp1-8 series. Three additional samples (CrSp9-11) were
120
produced to examine the effect of Fe# and non-stoichiometry on chromium-bearing
121
spinel spectra. Samples CrSp9 and 10 were synthesized following the procedure listed
122
above, but with the aim of fixing chromium content and varying iron content. Sample
123
CrSp11 was produced by adding surplus MgO powder to one of the samples from the
124
CrSp series (CrSp8) and resintering this composition.
125
Fragments of each synthetic spinel were mounted for electron microprobe
126
analysis, and the remainders were crushed and dry sieved to <45 μm particle size for
127
reflectance spectroscopy and Mössbauer measurements. The <45 μm particle size was
128
chosen in order to provide the most direct analog to remotely sensed lunar spinels, as
129
this particle size fraction dominates the optical properties of lunar soils at V-NIR
130
wavelengths (Pieters et al. 1993; Fischer 1995).
131
132
133
134
Electron microprobe analysis
Major element compositions of each sample were obtained using the Cameca
SX-100 electron microprobe at Brown University. Analyses were performed using 15 kV
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accelerating voltage, 20 nA beam current, and a focused beam. Point analyses were
136
spatially distributed throughout the entire sample in order to record any compositional
137
variation.
138
139
140
Mössbauer analysis
All samples were synthesized at the same oxygen fugacity. Mössbauer data
141
obtained at Mount Holyoke determined the coordination state of iron for two of the
142
spinel samples (CrSp3 and 4). The methods outlined in Jackson et al. (2014) are
143
identical for the samples presented in this paper, as the samples were analyzed
144
simultaneously. For a detailed description of the Mössbauer spectroscopy, see Jackson
145
et al. (2014).
146
147
Reflectance spectroscopy
148
Reflectance spectra of our synthetic spinels were acquired in RELAB at Brown
149
University with a spectral resolution ≤5 nm. Dry-sieved samples (<45 μm particle size)
150
were measured by both the Bidirectional Reflectance spectrometer (BDR), which
151
measures V-NIR wavelengths (300-2600 nm), and the Thermo Nicolet Nexus 870
152
Fourier Transform Infrared spectrometer (FTIR), which measures out to mid-infrared
153
(mid-IR) wavelengths (800-25000 nm). Each BDR and FTIR measurement employed an
154
incidence angle of 30° and emergence angle of 0° (Pieters and Hiroi, 2004). BDR and
155
FTIR data for each sample were spliced near 1000 nm using conventional RELAB
156
procedures in order to smoothly connect the two spectra. All reflectance spectra
157
presented in this paper will be available through the RELAB spectral database
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158
(www.planetary.brown.edu/relab). (RELAB sample directory names are reported in
159
Supplementary Table S1.)
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161
162
163
Spectral Analysis
V-NIR band strength is expected to increase with increasing iron and chromium
164
abundance in spinel. To measure this effect on the bands at 550, 1000, 2000, and 2800
165
nm, the strength of these bands in each sample spectrum was quantified. The spectrum
166
for each sample was first scaled so that the reflectance maximum between 1000–2000
167
nm (determined by a polynomial fit) equaled one. This procedure minimizes albedo
168
variations among samples to facilitate comparison of band strengths, making a further
169
continuum removal unnecessary. Then, a second-degree polynomial was fit in a least-
170
squares sense between 530-605, 825–1100, 1700–2200, and 2700–3000 nm. The
171
minimum reflectance value for each polynomial was identified, and the natural log of the
172
reflectance minimum [ln(reflectancemin)] for each band was calculated. The slope of the
173
correlation between ln(reflectancemin) and the concentration of the cation species
174
responsible for the band is termed the “reflectance coefficient.” The reflectance
175
coefficient is used to quantify the relationship between spinel composition and spectral
176
features.
177
Across the mid-IR spectral range (8000 – 25000 nm or 1250 – 400 cm-1),
178
radiation is commonly measured and reported in terms of emissivity, which can be
179
approximated from our data by subtracting the measured reflectance values from one
180
(Hapke, 1993). We report the mid-IR spectra in this way to facilitate comparison with
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181
previous laboratory studies. The primary spectral features in the mid-IR wavelengths
182
are 1) the Christiansen feature (CF) and 2) the restsrahlen bands (RB). The CF is
183
characterized by a primary emissivity maximum between ~10000 – 11000 nm (~900 –
184
1000 cm-1) with a secondary CF between ~15000 – 18000 nm (~550 – 650 cm-1) and is
185
diagnostic of mineralogy and bulk composition (Conel, 1969). The RB occur between
186
~12000 – 15000 nm (~650 – 850 cm-1) and ~15000 – 25000 nm (~400 – 650 cm-1) and
187
represent molecular vibrations related to stretching and bending motions. To determine
188
the wavelength position of these spectral features, a second-degree polynomial was fit
189
to each spectral feature in each spectrum following the approach used to calculate band
190
positions in the V-NIR spectral region. For the CF, a polynomial was fit to a portion of
191
the ~10000 – 11000 nm spectral range of each spectrum and the frequency of the
192
maximum emissivity value in the polynomial fit was used to represent the CF position.
193
The same method was used to find the position of the secondary CF in the ~15000 –
194
18000 nm spectral range. Diagnostic absorptions in the RB, four in total, were also fit to
195
determine the frequencies of the emissivity minimum values. The spectral range was
196
varied in order to best fit the emissivity and shape of each spectral feature in each
197
sample spectrum. Due to the non-unique nature of identifying the spectral features
198
using this methodology, the positions of spectral features can vary by ± 20 nm as the
199
spectral range and polynomial order are changed (Donaldson Hanna et al., 2012).
200
201
Results
202
Sample description and major element composition
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203
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The experimentally produced samples are porous and contain minor
204
compositional heterogeneities (Figure 2). Table 1 reports the average major element
205
compositions and standard deviations of our synthetic spinels. The predominant phase
206
in our samples is (Fe,Mg)(Cr,Al)2O4 spinel. For CrSp1-7, Cr# ranges from 1 to 29 with
207
minor variation in iron content (Fe# 8-11). CrSp9 and 10 have similar chromium
208
contents (Cr# 5 and 6, respectively) but vary significantly in iron content (CrSp9 Fe#
209
21.9, CrSp10 Fe# 30.4), as designed. Within-sample variations of spinel composition
210
are illustrated by the point analyses in Supplemental Table S2. In more Cr-rich samples,
211
excess Cr2O3 is present in phases <100 um in diameter (see Figure 2, a-b). Sample
212
CrSp8 (Cr# 14.2) is non-stoichiometric spinel with excess Al2O3. Sample CrSp11 (Cr#
213
14.1) contains minor amounts of periclase (MgO).
214
Mössbauer spectroscopy
215
The abundance of Fe3+ in spinel is positively correlated to the oxygen fugacity of
216
the system. Mössbauer spectroscopy measured the abundance of Fe3+ for samples
217
CrSp3 and 4 (Supplemental Table S3). Measured abundances show that Fe3+
218
comprises 10% and 6% of total iron in samples CrSp3 and CrSp4, respectively. These
219
measurements are consistent with Fe3+ abundances (3-14%) in spinels synthesized
220
between IW+1.6 and IW-0.3 by Hålenius et al. (2002), and confirm that reducing
221
conditions were obtained during each of the experimental runs. Because our spinels
222
were synthesized under low fO2 conditions, our results are relevant to spinel formation
223
on the Moon.
224
V-NIR spectroscopy
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Jackson et al. (2014) have detailed the effect of iron on visible-infrared spectra of
225
226
synthetic aluminate spinel with emphasis on the 700, 1000, 2000, and 2800 nm
227
absorption bands. The chromium-bearing spinel samples presented have an additional,
228
narrow (~130 nm wide) absorption near 550 nm (Figure 3). The 550 nm band is present
229
in spinels with Cr# as low as 1. Absorption strength increases with increasing chromium
230
content (Figure 4). Our results generate the linear correlation:
231
ln
0.030
0.007 ·
#
0.37
0.08,
0.9
232
with uncertainties reported in 95% confidence intervals. At the highest chromium
(1)
233
concentrations studied, the 550 nm feature appears less sensitive to changes in Cr#.
234
The strength and shape of the characteristic iron absorption features (at 1000, 2000,
235
and 2800 nm) are not affected by the presence of chromium in the experiments.
236
Likewise, iron (Figure 5) and magnesium (Figure 6) have little effect on the 550 nm
237
chromium band, though they exhibit significant effects on longer wavelength bands
238
(Jackson et al., 2014). Each sample spectrum displays a sharp decease in reflectance
239
at wavelengths below 550 nm. This effect is likely related to metal-oxygen charge
240
transfer absorptions associated with iron and chromium cations (Cloutis et al., 2008).
241
When the charge transfer absorptions are removed from the spectra near 550 nm, a
242
relationship between 550 nm feature depth and Cr# remains (Supplementary Figure
243
S3).
244
245
MID-IR spectroscopy
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246
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Full resolution laboratory emissivity (1 – reflectance) spectra for the CrSp spinel
247
series are plotted across the mid-IR spectral range (8000 – 25000 nm or 1250 – 400
248
cm-1) in Figure 7. Diagnostic spectral features include the primary Christiansen feature
249
(CF) observed near ~980 cm-1 (~10200 nm) and the secondary CF observed near ~620
250
cm-1 (~16100 nm). Mid-IR laboratory spectra show a systematic shift of the primary and
251
secondary CF positions to lower wavenumbers (longer wavelengths) as the Cr#
252
increases (as seen in Figures 7 and 8). Thus, the primary and secondary CF positions
253
can be used to distinguish between compositions of chromium-bearing spinel. The
254
equations fit to the CF positions (wavenumber, cm-1) are as follows:
255
1
.
2.2
0.1 ·
#
975
2,
0.996
(2)
256
2
.
0.6
0.2 ·
#
621
3,
0.873
(3)
257
A similar linear trend is observed between the primary and secondary CF positions and
258
Fe# in emissivity spectra of the olivine solid solution series (Hamilton, 2010) and the
259
synthetic suite of spinels with varying Fe# by Jackson et al. (2014). Four diagnostic
260
absorptions in the reststrahlen band (RB) regions were identified near ~865, 755, 700,
261
and 520 cm-1 (~11600, 13200, 14300, and 19200 nm). As seen in Figure 8, the RB
262
near ~860, 760 and 700 cm-1 also have linear relationships with Cr# as the diagnostic
263
band positions shift to lower wavenumbers (longer wavelengths), while the position of
264
RB4 near 520 cm-1 appears independent of Cr#. The equations fit to the RB positions
265
(wavenumber, cm-1) are as follows:
266
1
.
1.4
0.3 ·
#
863
4,
0.958
(4)
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2
267
3
268
.
.
2
0.8
2·
0.3 ·
#
760
#
16,
694
5,
0.653
0.865
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(5)
(6)
269
Thus, diagnostic absorption bands in the RB regions can also be used to distinguish
270
between chromium-bearing spinel compositions. Similar trends are observed between
271
RB absorption band positions and Fe# in spinels (Cloutis et al., 2004, Jackson et al.,
272
2014) and olivines (Hamilton, 2010). Uncertainties in regression equations are 95%
273
confidence intervals.
274
275
Discussion
276
Octahedral Cr3+ as cause for 550 nm absorption
277
The correlation between 550 nm band strength and Cr# in our samples, along
278
with previous studies of spinel spectra (Poole, 1964; Mao and Bell, 1975; Cloutis et al.,
279
2004), conclusively link the 550 nm absorption band to octahedral Cr3+ in spinel.
280
Tetrahedral Cr2+ is known to display an absorption band around 1300 nm (Greskovich
281
and Stubican, 1966; Mao and Bell, 1975; Cloutis et al., 2004). Absence of an absorption
282
band near 1300 nm in our spectral data, combined with spinel stoichiometry from our
283
compositional data, indicates the chromium in our samples is overwhelmingly present
284
as octahedral Cr3+.
285
Tetrahedral Fe2+ within spinel displays strong absorption bands in the near-
286
infrared wavelengths (Cloutis et al., 2004; Jackson et al., 2014), but does not produce
287
the absorption feature at 550 nm observed in our samples. Previous work at similar
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288
synthesis conditions (Jackson et al., 2014) has shown that increasing octahedral Fe2+
289
content increases the strength of the absorption band near 700 nm without giving rise to
290
any absorption bands near 550 nm. Because iron content is held constant for spinels
291
CrSp1-7, the increase in 550 nm band strength could only be due to Fe2+ (tetrahedral or
292
octahedral) provided octahedral Cr3+ was increasing the proportion of these iron
293
species. The current experiments demonstrate the addition of iron to chromium-bearing
294
spinels does not significantly increase the strength of the 550 nm absorption band
295
(Figure 5). Rather, the iron begins to produce an absorption shoulder near 700 nm.
296
Thus, the 550 nm feature appears to be solely a function of octahedral Cr3+.
297
Comparison of synthetic spinel to lunar sample 70002,7
298
Application of our results to remotely gathered data from the Moon requires
299
compositional and spectral similarities between lunar spinels and our synthetic samples.
300
Mao and Bell (1975) report a spectrum from a spinel found in Apollo sample 70002,7,
301
which is compositionally similar to the spinels synthesized in this study (Fe# 11.6 and
302
Cr# 4.29). The spinel spectrum from sample 70002,7 shows absorptions at 1000 and
303
2000 nm characteristic of Fe2+ (Mao and Bell, 1975; Cloutis et al., 2004; Jackson et al.,
304
2014). Additionally, the lunar spinel spectrum shows a narrow absorption band near 550
305
nm, which the authors attribute to octahedral Cr3+. The 550 nm absorption in the lunar
306
spinel spectrum is similar in strength and depth to the characteristic feature of our
307
synthetic chromium-bearing aluminate spinels (Figure 9). Furthermore, equation (1)
308
predicts Cr# 7 ± 3 from the spinel spectrum of Apollo sample 70002,7, within error of the
309
actual value of 4.3. Thus the experimental calibration can be used to quantify lunar
310
spinel compositions using the strength of the 550 nm spectral feature.
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311
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Mid-IR spectral systematics
Extensive data analysis is needed to characterize the mid-IR remote
313
observations of the lunar spinel locations identified by V-NIR techniques (Pieters et al.,
314
2011; Dhingra et al., 2011; Dhingra and Pieters, 2011; Bhattacharya et al., 2012; Kaur
315
et al., 2012; Lal et al., 2012; Yamamoto et al., 2013; Pieters et al., 2014). Our results
316
indicate that the mid-IR spectral range can be used to distinguish chromium-bearing
317
spinel compositions from: (1) one another based on the position of the CF and RB, (2)
318
Mg-spinel compositions with varying Fe#, and (3) silicate minerals found on the lunar
319
surface such as plagioclase, pyroxene, and olivine. As seen in Figure 8, diagnostic
320
spectral features of chromium-bearing spinels systematically shift to lower
321
wavenumbers (longer wavelengths) as the Cr# increases, and importantly, these shifts
322
are relatively large compared to those associated with changes in Fe# (c.f. Jackson et
323
al., 2014). Thus, for locations of pure chromium-bearing spinel on the lunar surface, the
324
CF and RB positions could distinguish the Cr# for that spinel. In addition, the CF and
325
RB positions of chromium-bearing spinel occur at lower wavenumbers (longer
326
wavelengths) that are distinct from the CF and RB positions of silicate minerals such as
327
plagioclase, pyroxene, and olivine. Lunar spinel locations have been identified based on
328
spectra dominated by the aluminate spinel signature, however V-NIR laboratory
329
analysis by Cheek and Pieters (2014) have shown that significant amounts of
330
plagioclase can be mixed into these spinel-rich units before the broad diagnostic
331
absorption band at 1250 nm of plagioclase is observed. Across the mid-IR spectral
332
range, mineral mixtures with rocks and coarse particulates have very high absorption
333
coefficients resulting in few multigrain interactions, and spectral features of the
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11/18
334
component minerals add linearly to produce the mixture spectrum (Christensen et al.,
335
1986). Thus, our laboratory mid-IR spectra of chromium-bearing spinels presented here,
336
along with detailed analysis of mid-IR remote observations of lunar spinel locations by
337
the Diviner Lunar Radiometer Experiment onboard NASA’s Lunar Reconaissance
338
Orbiter (LRO) and future hyperspectral data sets, could act to better constrain the
339
abundances and compositions of spinel in these spinel-rich areas. In particular, Diviner
340
has three narrow spectral channels near 8000 nm (7810, 8280, and 8550 nm) and one
341
wider spectral channel covering the 12500 – 25000 nm spectral range, which can be
342
used to characterize the wavelength position of the CF and the overall shape of the RB
343
region. These four Diviner spectral channels could be used to easily distinguish
344
between Cr-rich, intermediate-Cr, and Cr-poor spinel compositions, however laboratory
345
emissivity measurements of our synthetic samples under simulated lunar conditions are
346
needed to further refine the degree to which Diviner would be able to distinguish
347
between spinel compositions (e.g., Cr#).
348
349
Implications
350
The connection between a narrow 550 nm absorption band and octahedral Cr3+
351
in the spinel structure provides a tool by which to identify chromium in remotely sensed
352
spinel. Because these experiments were conducted at fO2 ~IW-1, the results are
353
applicable to any planetary surface with similar redox conditions. Below, we explore the
354
implications of our laboratory investigation for the petrologic interpretation of specific
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11/18
355
spinel-rich occurrences on the Moon: pink spinel anorthosites in the highlands, and dark
356
mantle deposits at Sinus Aestuum.
357
Pink Spinel Anorthosite (PSA) lithology
358
The lack of any observed absorption peak near 550 nm in spectra from both M3
359
and SP datasets of spinel-bearing lithologies suggests there is little octahedral Cr3+ in
360
the remotely sensed spinel, potentially Cr# <1. Spinel in lunar clasts believed to
361
represent the Mg-rich spinel all have Cr# >1 which, by our results, would produce a 550
362
nm absorption feature (Papike et al., 1998; Gross and Treiman, 2011; Treiman and
363
Gross, 2015; Wittman et al., 2015).
364
Assuming the lack of observable 550 nm bands associated with PSA is not a
365
result of other processes, it indicates that the remotely sensed spinels are very Cr-poor
366
and Al-rich. Highly aluminate spinels are typically associated with Mg-suite materials
367
and can be produced by assimilation of plagioclase-rich lunar crust by Mg-suite parental
368
magmas (Prissel et al., 2014). The petrology of the Mg-suite rocks indicates their
369
parental magmas contained high normative abundances of plagioclase and were
370
relatively Cr-poor (Elardo et al., 2011), providing a link between Mg-suite magmatism
371
and PSA observations. This link is strengthened by the low Fe# associated with the Mg-
372
suite parental magmas and PSA spinel (Jackson et al., 2014). Thus, PSA lithologies
373
plausibly represent locations where substantial amounts of crustal material were
374
assimilated into Mg-suite magmas. Alternatively, the Cr-poor and Al-rich spinel
375
composition could be produced from hybridized mantle melts (Longhi et al., 2010) or
376
impact melting (Gross et al., 2014).
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Dark Mantle Deposits at Sinus Aestuum
Spectral data on pyroclastic deposits at Sinus Aestuum contain a “visible feature”
379
around 600-700 nm (Sunshine et al., 2010, 2014; Yamamoto et al., 2013), but no
380
obvious absorptions at shorter wavelengths. The 600-700 nm band center is clearly
381
offset from band centers related to octahedral Cr3+, a fundamental cation species
382
required to stabilize chromite components within spinel. Thus, our results argue against
383
previous suggestions that the Dark Mantle Deposits at Sinus Aestuum contain large
384
chromite components because the characteristic octahedral Cr3+ spinel absorption is
385
absent (Sunshine et al., 2010, 2014; Yamamoto et al., 2013).
386
Rather, we suggest that the spinels observed at Sinus Aestuum are dominantly
387
aluminate spinels that are distinctly more Fe-rich compared to the remaining remote
388
observations of PSA spinels. This suggestion is in accordance with previous
389
interpretations of the Sinus Aestuum spinel spectra (Yamamoto et al., 2013), as well as
390
the Fe-rich (>16 wt% FeO) pyroclastic glasses returned from the Moon (e.g. Delano,
391
1986). However, Cr-rich spinels are expected to form during typical mare basalt or lunar
392
picritic liquid crystallization (Elkins-Tanton et al., 2003). In order to stabilize low Cr#
393
spinels, we suggest the pyroclastic materials at Sinus Aestuum assimilated a relatively
394
large amount of plagioclase, similar to the model proposed for PSA (above), leading to
395
a decreased Cr# prior to eruption. Again, this suggests that Sinus Aestuum region
396
experienced a relatively protracted magmatic history with multiple pulses of magmatism,
397
as these conditions favor assimilation of country rock.
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398
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Select spectra at Sinus Aestuum are reported to exhibit a 1300 nm absorption
399
band (Yamamoto et al., 2013), and this band has been attributed to tetrahedral Cr2+
400
(Cloutis et al., 2004). It is possible that tetrahedral Cr2+ is present in the Sinus Aestuum
401
spinels, but given the lack of evidence for octahedral Cr3+, the affinity of spinel for
402
octahedral Cr3+, and the observation of abundant octahedral Cr3+ in lunar spinels
403
(Haggerty, 1971, 1972, 1973, 1977), this would imply an extremely reduced oxygen
404
fugacity for the Sinus Aestuum pyroclastic materials.
405
Alternative explanations for the lack of an observed 550 nm absorption could
406
include that (1) the remote detection limits are very high (perhaps related to the
407
decreased signal/noise ratio of M3 and SP at short wavelengths), (2) space weathering
408
has preferentially muted the 550 nm band, or (3) mixing with other materials has
409
preferentially muted the 550 nm band. Gross et al. (2015) demonstrated that the Cr3+
410
absorption near 550 nm remains robust after simulated space weathering, indicating
411
that space weathering is not a likely cause for the lack of 550 nm bands in remotely
412
sensed lunar spinels. Further, laboratory spectra taken of spinel-plagioclase mixtures
413
demonstrate that the 550 nm band for a spinel with Cr# 0.1 is detectable in laboratory
414
settings, even at low spinel abundance (~15 vol%) (Cheek and Pieters, 2014). V-NIR
415
and thermal mixture modeling, combined with thorough analysis of M3 and SP
416
signal/noise ratios at short wavelengths, should help to refine this constraint. Future
417
work to determine the effect of lower oxygen fugacity, higher chromium contents, and
418
space weathering across a range of wavelengths will improve compositional constraints
419
based on the 550 nm feature.
420
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421
Acknowledgements:
422
The authors thank Taki Hiroi for assistance with spectral measurements and Joseph
423
Boesenberg for assistance with EMP measurements. We thank E.A. Cloutis and S.
424
Yamamoto for their thoughtful reviews of the manuscript. Additional thanks go to Reid F.
425
Cooper who assisted in the early stages of this project. Support for this work was
426
provided through NLSI (NNA09DB34A) and SSERVI (NNA13AB014).
427
428
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References:
430
431
Bhattacharya, S., Chauhan, P., and Ajai [no initial] (2012) Discovery of orthopyroxene-
432
olivine-spinel assemblage from the lunar nearside using Chandrayaan-1 Moon
433
Mineralogy Mapper data. Current Science, 103, 21–23.
434
Bhattacharya, S., Saran, S., Dagar, A. Chauhan, P., Chauhan, M., Ajai [no initial], and
435
Kumar, A.S.K. (2013) Endogenic water on the Moon associated with non-mare silicic
436
volcanism: implications for hydrated lunar interior. Current Science, 105, 685–691.
437
Cheek, L.C., and Pieters, C.M. (2014) Reflectance spectroscopy of plagioclase and
438
mafic mineral mixtures: Implications for characterizing lunar anorthosites remotely.
439
American Mineralogist, 99, 1871–1892.
440
Christensen, P. R. (1986) The spatial distribution of rocks on Mars. Icarus, 68, 217-238.
441
Cloutis, E.A., Sunshine, J.M., and Morris, R.V. (2004) Spectral reflectance-
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
442
compositional properties of spinels and chromites: Implications for planetary remote
443
sensing and geothermometry. Meteoritics & Planetary Science, 39, 545–565.
444
Cloutis, E.A., McCormack, K.A., Bell, J.F. III, Hendrix, A.R., Bailey, D.T., Craig, M.A.,
445
Mertzman, S.A., Robinson, M.S., and Riner, M.A. (2008) Ultraviolet spectral reflectance
446
properties of common planetary minerals. Icarus, 197, 321-347.
447
Conel, J. E. (1969) Infrared emissivities of silicates: Experimental results and a cloudy
448
atmospheric model of spectral emission from condensed particulate mediums. Journal
449
of Geophysical Research, 74, 1614-1634.
450
Delano, J.W., (1986) Pristine Lunar Glasses: Criteria, Data, and Implications. Journal of
451
Geophysical Research, 91, B4, D201 – D213.
452
Dhingra, D., and Pieters, C.M. (2011) Mg-Spinel rich lithology at crater Copernicus.
453
Annual Meeting of the Lunar Exploration Analysis Group, 2024.
454
Dhingra, D., Pieters, C.M., Boardman, J., Head, J.W. III, Isaacson, P.J., Taylor, L.A.,
455
and M3 Team (2011a) Theophilus Crater: Compositional diversity and geological
456
context of Mg-spinel bearing central peaks. Lunar and Planetary Science Conference,
457
42, 2388.
458
Dhingra, D., Pieters, C.M., Boardman, J.W., Head, J.W., Isaacson, P.J., and Taylor,
459
L.A. (2011b) Compositional diversity at Theophilus crater: Understanding the geologic
460
context of Mg-spinel bearing central peaks. Geophysical Research Letters, 38, L11201.
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
461
Donaldson Hanna, K. L., I. R. Thomas, N. E. Bowles, B. T. Greenhagen, C. M. Pieters,
462
J. F. Mustard, C. R. M. Jackson, and M. B. Wyatt (2012) Laboratory emissivity
463
measurements of the plagioclase solid solution series under varying environmental
464
conditions, Journal Geophysical Research, 117, E11, doi:10.1029/2012JE004184.
465
Donaldson Hanna, K. (2013) Compositional diversity of the Moon’s anorthositic crust:
466
Integrating remote observations with laboratory measurements of analog materials
467
under simulated lunar conditions, p. 306. Ph.D. thesis, Brown University, Providence,
468
Rhode Island.
469
Elardo, S.M., Draper, D.S., and Shearer, C.K. Jr. (2011) Lunar Magma Ocean
470
crystallization revisited: Bulk composition, early cumulate mineralogy, and the source
471
regions of the highlands Mg-suite. Geochimica et Cosmochimica Acta, 75, 3024–3045.
472
Elkins-Tanton, L.T., Chatterjee, N., and Grove, T.L. (2003) Experimental and
473
petrological constraints on lunar differentiation from the Apollo 15 green picritic glasses.
474
Meteoritics and Planetary Science, 38, 515-527.
475
Fischer, E.M. (1995) Quantitative compositional analysis of the lunar surface from
476
reflectance spectroscopy: Iron, aluminum, and a model for removing the optical effects
477
of space weathering. Ph.D. dissertation, Brown University, Providence, Rhode Island.
478
Greskovich G. and Stubican V. S. 1966. Divalent chromium in magnesium-chromium
479
spinels. Journal of Physics and Chemistry of Solids 27:1379–1384.
480
Gross, J., and Treiman, A.H. (2011) Unique spinel-rich lithology in lunar meteorite ALHA
481
81005: Origin and possible connection to M3 observations of the far- side highlands.
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
482
Journal of Geophysical Research, 116, E10009,
483
http://dx.doi.org/10.1029/2011JE003858.
484
Gross, J., Isaacson, P.J., Treiman, A.H., Le, L., and Gorman, J.K. (2014) Spinel-rich
485
lithologies in the lunar highland crust: Linking lunar samples with crystallization
486
experiments and remote sensing. American Mineralogist, 99, 1849-1859.
487
Gross, L., Gillis-Davis, J., Isaacson, P.J., and Le, L. (2015) How rich is rich? Placing
488
constraints on the abundance of spinel in the pink spinel anorthosite lithology on the
489
Moon through space weathering. Lunar and Planetary Science Conference, 46, 2642.
490
Haggerty, S.E., 1971. Compositional variations in lunar spinels. Nature, 233 (43), 156–
491
160.
492
Haggerty, S.E., 1972. Apollo 14: subsolidus reduction and compositional variations of
493
spinels. In: 3rd Lunar Science Conference Proceedings, 1, pp. 305–332.
494
Haggerty, S.E., 1973. Luna 20: mineral chemistry of spinel, pleonaste, chromite, ul-
495
vo spinel, ilmenite and rutile. Geochimica et Cosmochimica Acta, 37 (4), 857–867.
496
Haggerty, S.E., 1977. Luna 24: opaque mineral chemistry of gabbroic and basaltic
497
fragments from mare crisium. Geophysical Reserach Letters, 4 (10), 489–492.
498
Hålenius, U., Skogby, H., and Andreozzi, G.B. (2002) Influence of cation distribution on
499
the optical absorption spectra of Fe3+-bearing spinel s.s. hercynite crystals: evidence
500
for electron transitions in VIFe2+–VIFe3+ clusters. Physics and Chemistry of Minerals,
501
29, 39–330.
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
502
Hamilton, V. E. (2010) Thermal infrared (vibrational) spectroscopy of Mg-Fe olivines: A
503
review and applications to determining the composition of planetary surfaces. Chemie
504
der Erde, 70, 7-33
505
Hapke, B. (1993) Theory of Reflectance and Emittance Spectroscopy, Cambridge
506
University Press, New York.
507
Jackson, C.R.M., Cheek, L.C., Williams, K.B., Donaldson Hanna, K., Pieters, C.M.,
508
Parman S.W., Cooper, R.F., Dyar, M.D., Nelms, M., and Salvatore, M.R. (2014) Visible-
509
infrared spectral properties of iron-bearing aluminate spinel under lunar-like redox
510
conditions. American Mineralogist, 99, 1821–1833.
511
Kaur, P., Chauhan, P., Bhattacharya, S., Ajai [no intial], and Kumar, A.S.K. (2012)
512
Compositional diversity at Tycho Crater: Mg-spinel exposures detected from Moon
513
Mineralogical Mapper (M3) data. Lunar and Planetary Science Conference, 43, 1434.
514
Kaur, P., Chauhan, P., and Ajai [no initial] (2013a) Detection of Mg-spinel exposures
515
from the anorthositic terrain surrounding Mare Ingenii on the far side of the Moon. Lunar
516
and Planetary Science Conference, 44, 1547.
517
Kaur, P., Chauhan, P., and Ajai [no initial] (2013b) Exposures of Mg-spinel on an
518
evolved silicic lithology Hansteen Alpha on the Moon. Lunar and Planetary Science
519
Conference, 44, 1348.
520
Kaur, P., and Chauhan, P. (2014) Detection of spinels exposures from some near side
521
locations of the Moon. Lunar and Planetary Science Conference, 45, 2059.
522
Lal, D., Chauhan, P., Shah, R.D., Bhattacharya, S., Ajai [no initial], and Kumar, A.S.K.
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
523
(2011) Identification of spinel group of minerals on central peak of crater Theophilus.
524
Lunar and Planetary Science Conference, 42, 1339.
525
Lal, D., Chauhan, P., Shah, R.D., Bhattacharya, S., Ajai [no initial], and Kirankumar,
526
A.S. (2012) Detection of Mg spinel lithologies on central peak of crater Theophilus using
527
Moon Mineralogy Mapper (M3) data from Chandrayaan-1. Journal of Earth System
528
Science, 121, 847–853.
529
Longhi, J., Durand, S.R., and Walker, D. 2010. The pattern of Ni and Co abundances in
530
lunar olivines. Geochimica et Cosmochimica Acta, 74, 784 - 798.
531
Mao, H.K., and Bell, P.M. (1975) Crystal-field effects in spinel: oxidation states of iron
532
and chromium. Geochimica et Cosmochimica Acta, 39, 865–866, IN1–IN2, 867–874.
533
Navrotsky, A., and Kleppa, O.J. (1967) Thermodynamics of cation distributions in simple
534
spinels. Journal of Inorganic & Nuclear Chemistry, 29, 2701–2714.
535
Papike, J.J., Ryder, G., and Shearer, C.K. Jr. (1998) Lunar Samples. Reviews in
536
Mineralogy and Geochemistry, 36, 5.1-5.234.
537
Pieters, C.M., Fischer, E.M., Rode, O., and Basu, A. (1993) Optical effects of space
538
weathering: The role of the finest fraction. Journal of Geophysical Research, 98, 20817–
539
20824.
540
Pieters, C.M., and T. Hiroi (2004) RELAB (Reflectance Experiment Laboratory): A
541
NASA Multiuser Spectroscopy Facility. Lunar and Planetary Science Conference, 35,
542
Abstract 1720.
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
543
Pieters, C.M., Boardman, J., Buratti, B., Clark, R., Combe, J.-P., Green, R., Goswami,
544
J. N., Head, J.W. III, Hicks, M., Isaacson, P., and others. (2010) Identification of a new
545
spinel-rich lunar rock type by the Moon Mineralogy Mapper (M3). Lunar and Planetary
546
Science Conference, 41, 1854.
547
Pieters, C.M., Besse, S., Boardman, J., Buratti, B., Cheek, L., Clark, R.N., Combe, J.P.,
548
Dhingra, D., Goswami, J.N., Green, R.O., and others. (2011) Mg-spinel lithology: A new
549
rock type on the lunar farside. Journal of Geophysical Research-Planets, 116, E00G08.
550
Pieters C.M., Donaldson Hanna, K., Cheek, L., Dhingra, D., Moriarty, D., Parman, S.,
551
Jackson, C., and Prissel, T. (2013) Compositional Evolution of the early lunar crust:
552
Observed diverse mineralogy of the upper and lower crust. Lunar and Planetary
553
Science Conference, 44, 2545.
554
Pieters, C.M., Donaldson-Hanna, K., Cheek, L., Dhingra, D., Prissel, T., Jackson, C.,
555
Moriarty, D., Parman, S., and Taylor, L.A. (2014) The distribution of Mg-spinel across
556
the Moon and constraints on crustal origin. American Mineralogist, 99, 1893–1910.
557
Poole C. P. 1964. The optical spectra and color of chromium containing solids. Journal
558
of Physics and Chemistry of Solids 25: 1169–1182.
559
Prissel, T.C., Parman, S.W. Jackson, C.R., Rutherford, M.J., Hess, P.C., Head, J.W.,
560
Cheek, L., Dhingra, D., and Pieters, C.M. (2014) Pink Moon: The petrogenesis of pink
561
spinel anorthosites and implications concerning Mg-suite magmatism. Earth and
562
Planetary Science Letters, 403, 144–156, http://dx.doi.org/10.1016/j. epsl.2014.06.027.
563
Sun, Y., Lin, L., and Zhang, Y.Z. (2013) Detection of Mg-spinel bearing central peaks
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
564
using M3 images. Lunar and Planetary Science Conference, 44, 1393.
565
Sunshine, J., Besse, S., Petro, N., Pieters, C., Head, J., Taylor, L., Klima, R., Isaacson,
566
P., Boardman, J., and Clark, R. (2010) Hidden in plain sight: Spinel-rich deposits on the
567
nearside of the Moon as revealed by Moon Mineralogy Mapper (M3). Lunar and
568
Planetary Science Conference, 41, Abstract 1508.
569
Sunshine, J., Petro, N., Besse, S., Gaddis, L.R. (2014) Widespread Exposures of Small
570
Scale Spinel-Rich Pyroclastic Deposits in Sinus Aestuum. Lunar and Planetary Science
571
Conference, 45, Abstract 1777.
572
Taylor, L.A., and Pieters, C.M. (2013) Pink-spinel anorthosite formation: Considerations
573
for a feasible petrogenesis. Lunar and Planetary Science Conference, 44, 2785.
574
Treiman, A.H., and Gross, J. (2015) A rock fragment related to the magnesian suite in
575
lunar meteorite Allan Hills (ALHA) 81005. American Mineralogist, 100, 414-426.
576
Wittman, A., Korotev, R.L., and Jolliff, B.L. (2015) Lunar Mantle Spinel in Dhofar 1528?
577
Lunar and Planetary Science Conference, 46, 1832.
578
Yamamoto, S., Nakamura, R., Matsunaga, T., Ogawa, Y., Ishihara, Y., Morota, T.,
579
Hirata, N., Ohtake, M., Hiroi, T., and Yokota, Y. (2013) A new type of pyroclastic deposit
580
on the Moon containing Fe-spinel and chromite. Geophysical Research Letters, 40,
581
4549–4554.
582
583
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
584
11/18
Figure captions:
585
586
Figure 1, V-NIR spectra of terrestrial and lunar chromium-bearing spinels. Note the
587
strong, broad, characteristic spinel absorptions near 2000 and 2400 nm. Terrestrial
588
samples S117, S124, and S125 (Cloutis et al., 2004) illustrate the effect of Fe2O3 on
589
spinel spectra, most notably in shorter wavelengths (<1200 nm). In comparison, the
590
spinel spectrum from lunar sample 70002,7 (Mao and Bell, 1975) exhibits a prominent
591
band only at 550 nm in the shorter wavelengths and is substantially brighter between
592
600 and 1000 nm. The sharp 550 nm feature also appears in the terrestrial spectra -
593
though slightly muted - and is here attributed to octahedral Cr3+. Figure legend identifies
594
sample name and oxide weight percent (wt%) for species of interest. Spectral
595
reflectance scaled as described in text.
596
597
Figure 2, Back-scattered electron (BSE) images showing minor heterogeneity of
598
experimental products. a) Porous CrSp3 grain: light grey represents spinel phase; bright
599
spots caused by excess Cr2O3. b) Enlarged BSE image of CrSp3 (image area is
600
highlighted by the inset in a). c) CrSp2: light grey represents spinel phase. d) CrSp11:
601
dark grey grain portions at top of image are periclase (MgO).
602
Figure 3, V-NIR spectra of CrSp1-7 (<45 µm grain size) with ~ 5 wt % FeO. All samples
603
display a relatively narrow band centered near 550 nm associated with octahedral Cr3+.
604
Separate bands centered at 1000, 2000, and 2800 nm are also present and associated
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
605
with Fe. Figure legend identifies sample name and Cr#. Spectral reflectance scaled as
606
described in text. Unscaled spectra plot available in supplement.
607
608
Figure 4, Correlation between ln(reflectancemin) for the 550 nm band and Cr# at <45 µm
609
particle size for samples. The ln(reflectancemin) parameter is inversely proportional to
610
band strength. Values of ln(reflectancemin) for the 550 nm band correlate linearly with
611
Cr# [slope = -0.030 ± 0.007 (95% confidence), r2 = 0.9], but the regression does not
612
pass through the origin [intercept = -0.37 ± 0.08 (95% confidence)]. This likely reflects a
613
small amount of darkening local to the 550 nm region related to other absorbing
614
species.
615
616
Figure 5, Effect of Fe on V-NIR spectra of chromium-bearing spinel: Samples CrSp5,
617
CrSp9, and CrSp10 show the V-NIR spectral effects of increasing bulk Fe content on
618
samples with Cr# ~ 5. Figure legend identifies sample name, Cr#, and Fe#. Spectral
619
reflectance scaled as described in text. Unscaled spectra plot available in supplement.
620
621
622
Figure 6, Effect of MgO-rich non-stoichiometry on V-NIR spectra:
Sample CrSp11
623
(solid spectrum) was prepared by adding an additional 4.1 wt % MgO to the starting
624
oxides of CrSp8 (dashed spectrum). The addition of MgO shifts the non-stoichiometry
625
of the spinel away from excess Al2O3. Sample CrSp11 has severely weakened 2000
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
626
and 2800 nm bands but a strengthened 1000 nm band compared to CrSp8. Spectral
627
reflectance scaled as described in text.
628
629
Figure 7, Mid-IR spectra from select samples (<45 µm grain size). Spectra are offset
630
for clarity and vertical lines highlight the positions of identified features. The primary
631
Christiansen feature is near ~980 cm-1 (~10200 nm) and the secondary CF is near ~620
632
cm-1 (~16100 nm). Reststrahlen band regions are located near ~865, 755, 700, and 520
633
cm-1 (~11600, 13200, 14300, and 19200 nm).
634
spectrum for CrSp5 is anomalous compared to the other mid-IR spectra of the CrSp
635
series but the mid-IR CF and RB feature positions are consistent with the remaining
636
spectra.
The spectral contrast in the mid-IR
637
638
Figure 8, Mid-IR spectral systematics from the CrSp series (<45 µm grain size). The
639
positions of Christiansen features 1 (a) and 2 (b) are negatively correlated to Cr#. Four
640
reststrahlen bands are identified (Absorptions 1-4, c-f). The positions of Absorption 1, 2
641
and 3 are negatively correlated to Cr#. The position of Absorption 4 appears
642
independent of Cr#.
643
644
Figure 9, Sample spectra (CrSp5 and CrSp9) comparison with spectra of spinel in
645
Apollo sample 70002,7 from Mao and Bell (1975). Figure legend identifies sample
646
name, Cr#, and Fe#. The 550 nm band shape in the lunar spinel spectrum is identical to
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
This is a preprint, the final version is subject to change, of the American Mineralogist (MSA)
Cite as Authors (Year) Title. American Mineralogist, in press.
(DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2016-5535
11/18
647
that of our synthetic analogs, and all three samples contain roughly the same amount
648
chromium. Spectral reflectance scaled as described in text.
649
650
651
Supplementary Figure Captions
Figure S1, V-NIR spectra of CrSp1-7 (<45 µm grain size) with ~ 5 wt % FeO. All
652
samples display a relatively narrow band centered near 550 nm associated with
653
octahedral Cr3+. Separate bands centered at 1000, 2000, and 2800 nm are also present
654
and associated with Fe. Figure legend identifies sample name and Cr#.
655
656
Figure S2, Effect of Fe on V-NIR spectra of chromium-bearing spinel: Samples CrSp5,
657
CrSp9, and CrSp10 show the V-NIR spectral effects of increasing bulk Fe content on
658
samples with Cr# ~ 5. Figure legend identifies sample name, Cr#, and Fe#.
659
Figure S3, Continuum-removed V-NIR spectra (a) of CrSp1-7 (<45 µm grain size) with
660
~ 5 wt% FeO. A continuum fit between 500 nm and 650 nm was removed to isolate the
661
550 nm feature from the short-wavelength steepening observed in the original spectra.
662
The resulting correlation between ln(reflectancemin) for the 550 nm band and Cr# differs
663
from the original regression line, but maintains that the ln(reflectancemin) parameter is
664
inversely proportional to band strength [-0.0075 Cr# + 0.8621, r2 = 0.9].
665
Always consult and cite the final, published document. See http://www.minsocam.org or GeoscienceWorld
Table 1. Averaged compositions of experimental starting materials reported in wt% oxides.
na
SiO2
Al2O3
S.D.
Cr2O3
S.D.
FeOb
S.D.
MnO
CrSp1
9
--
--
68
1
1
1
5
2
CrSp2
6
--
--
67
2
3.1
7
5.5
CrSp3
7
0.08
6
--
67
1
3
1
CrSp4
6
0.1
5
--
67
2
4
CrSp5
9
0.12
3
--
64
4
CrSp6
11
0.09
6
--
50
CrSp7
8
0.2
1
--
CrSp8
5
--
CrSp9
9
CrSp10
CrSp11
MgO
S.D.
CaO
--
26
1
--
2
--
25.2
5
4
1
--
25
1
4
1
--
5
3
5.1
3
5
21
4
4.5
45
3
27
3
--
58.2
6
14
--
--
63
2
10
--
--
62
10
0.16
--
56.9
S.D.
2
TiO2
S.D.
Total
S.D.
--
100
2
--
--
100
2
1
n.a.
--
99.7
7
24.6
8
n.a.
--
100
1
--
25.0
9
--
--
99
2
3
--
23.7
9
--
--
100
2
5.0
3
0.08
2
22.9
4
n.a.
--
100
1
1
5.0
4
0.05
3
21.9
8
n.a.
--
100
3
5
2
10.9
3
--
21.7
3
n.a.
--
101.1
8
2
5
2
14.5
2
--
18.5
7
--
--
100
2
7
13.9
6
2.7
6
--
26.0
4
--
0.05
100
1
S.D.
S.D.
NiO
S.D.
4
Note : S.D. denotes 2σ standard deviation on the last significant digit reported. Cr# = [Cr/(Cr + Al)] x 100; Fe# = [Fe/(Fe + Mg)] x 100. Analyses below detectable limits denoted by "--"
a
Number of analyses.
b
FeO = total iron.
Cr#
S.D.
Fe#
S.D.
1
1
11
4
3.0
7
10.9
2
3
1
8
3
4
1
9
2
5
4
10.2
8
22
5
9.6
8
29
4
11.0
6
14.2
9
11.3
5
6
2
21.9
6
5
2
30.4
8
14.1
6
5
1
below detectable limits denoted by "--". n.a. = not analyzed.
Figure 2